The present invention relates to proteins capable of binding the E2 envelope protein of hepatitis C virus (HCV) and to processes for production and purification.
The invention also relates to the use of the proteins in therapy and diagnosis and to pharmaceutical compositions and diagnostic kits for such uses. The invention also relates to a process for screening putative molecules for competition with HCV for receptor binding. The invention also relates to an animal model for HCV infection.
HCV (previously known as Non-A Non-B hepatitisxe2x80x94NANBV)is a positive sense RNA virus of about 10000 nucleotides with a single open reading frame encoding a polyprotein of about 3000 amino acids. Although the structure of the virus has been elucidated by recombinant DNA techniques (1, 2), the virus itself has not been isolated and the functions of the various viral proteins produced by proteolysis of the polyprotein have only been inferred by analogy with other similar viruses of similar genomic organisation (3).
The viral proteins are all available in recombinant form, expressed in a variety of cells and cell types, including yeast, bacteria, insect and mammalian cells (4,5).
Two proteins, named E1 and E2 (corresponding to amino acids 192-383 and 384-750 respectively) have been suggested to be external proteins of the viral envelope which are responsible for the binding of virus to target cells (3).
HCV research is hindered very considerably by the limited host range of the virus. The only reliable animal model for HCV infection is the chimpanzee and it is not possible to propagate HCV in tissue culture.
In our copending International patent application PCT/IB95/00692, we describe a method employing flow cytometry to identify cells carrying the HCV receptor. We have shown that, by labelling cells with recombinant E2 envelope protein, it is possible to sort cells using flow cytometry, isolating those cells capable of specific binding to the E2 and therefore potentially carrying the HCV receptor. Employing this technique, we have identified a protein capable of binding to the E2 envelope protein of HCV which we believe to be the receptor for HCV, thereby enabling overcoming many problems in the art.
According to the present invention, there is provided a protein having a molecular weight of about 24 kD and capable of specifically binding to a protein of hepatitis C virus, or a functionally equivalent variant or fragment thereof.
It will be understood by the skilled person that molecular weights measured as described below using electrophoresis are inherently subject to interpretation since they are measured relative to standard molecular weight markers. However, in the context of this specification the expression xe2x80x9c24 kdxe2x80x9d is unambiguous when read in context, since only one such protein is obtained by following the processes described below with the defined characteristic of binding to hepatitis C virus.
A significant characterising feature of the protein according to the present invention is its ability to bind specifically to an HCV protein, preferably an envelope protein, particularly the E2 protein.
On the basis of this specificity and other features described below, we infer that the 24 kd protein of the invention is a cellular receptor for HCV.
We have shown that the protein is ubiquitous in humans amongst the cell types we have tested, paralleling the situation found for many other viruses of this type (such as vaccinia virus and influenza virus).
We have shown that the protein is species specific in a manner which matches the species specificity of HCV itself.
Our experiments have shown that the 24 kd protein is functionally unglycosylated. Treatment with glycosidases does not affect the ability of the 24 kd protein to bind to the E2 protein and does not appear significantly to reduce the molecular weight. We infer therefore that, if the protein is glycosylated at all, glycosylation must be restricted to a small number of sugar moieties and is not necessary for functional activity of the protein.
Our experiments have also shown that the protein is a transmembrane protein, again suggesting that it is a cellular receptor.
Finally, experiments with cell lines hyperexpressing the protein indicate that such cells are prone to aggregation suggesting that the protein may be an adhesion molecule of some form.
The 24 kd protein may be in its naturally occurring form, albeit isolated from its native environment, or may be modified, provided that it retains the functional characteristic of at least binding to the E2 protein of HCV. For example, the 24 kd protein may be modified chemically to introduce one or more chemical modifications to the amino acid structure. It may include modifications of the amino acid sequence involving one or more insertions, deletions or replaced amino acids. It may, for example, be truncated by the removal of a functional part of the transmembrane domain to facilitate ready production by recombinant DNA means in a suitable mammalian host cell (6).
The protein of the present invention may be purified from cells exhibiting binding to an HCV protein, such as the E2 protein.
According to the present invention there is provided a process for the preparation of a protein according to the invention or a functionally equivalent variant or fragment thereof comprising the step of culturing cells exhibiting binding to an HCV protein and purifying from a cell preparation a protein according to the invention.
The cells may be transformed or untransformed mammalian cells and are suitably human cells.
The cells may be screened for binding to an HCV protein using fluorescence flow cytometry or any other suitable assay. For example, the present description provides the information necessary to produce the 24 kd protein or a functionally equivalent variant or fragment thereof which then itself be used to assay for further cells carrying the protein.
The cell preparation may be a cell membrane preparation but is preferably a plasma cell membrane preparation.
Preferably the cells are selected and cloned to provide hyperexpression of the protein of the present invention.
We have discovered that the protein is precipitated by ammonium sulphate at between 33 and 50% of saturation.
Preferably, therefore, the cell preparation is subjected to an ammonium sulphate precipitation purification step employing ammonium sulphate at between 33 and 50%. Suitably a first precipitation is conducted at less than 33% and precipitated material discarded followed by precipitation of the desired material at between 33 and 50%, most preferably 50%.
Preferably, the purification involves at least one step of hydrophobic interaction chromatography.
We have also discovered that the protein is stable to acetone precipitation, thereby providing a still further characterisation and a useful purification process step.
Most preferably in optimised form, the process of purification comprises the steps of:
i) preparing a plasma cell membrane preparation of mammalian cells selected for hyperexpression of the 24 kd protein of the invention,
ii) subjecting the preparation to ammonium sulphate precipitation at less than 33% saturation and retaining the supernatant,
iii) subjecting the supernatant to ammonium sulphate precipitation at between 33 and 50% saturation and retaining the precipitate, and
iv) resuspending the precipitate and subjecting it to hydrophobic interaction chromatography
As an alternative to purification from wild-type cell lines, the protein of the invention or a functionally equivalent variant or fragment thereof may be made by any suitable synthetic process including chemical synthesis. Suitably, the protein or a functionally equivalent variant or fragment thereof is made by expression of a gene encoding the protein in a suitable host cell or animal.
According to a further aspect of the invention, there is provided a method for treating an infection of HCV comprising administering to a patient an amount of the protein of the invention or a functionally equivalent variant or fragment thereof effective to reduce the infectivity of the virus.
Since the infection mechanism of HCV appears to depend, in part, upon the availability of a cell surface receptor, making available a soluble form of the protein of the invention will act as an antagonist of binding of HCV to the cellular receptor thus reducing or preventing the infection process and thereby treating the disease.
A suitable form of the protein of the invention might comprise, for example, a truncated form of the protein from which the transmembrane domain has been removed either by a protein cleavage step or, by design, in a chemical or recombinant DNA synthesis.
Alternatively, a hybrid particle comprising at least one particle-forming protein, such as hepatitis B surface antigen or a particle-forming fragment thereof, in combination with the protein of the invention or a functionally equivalent variant or fragment thereof could be used as an antagonist of binding of HCV to the cellular receptor.
According to a further aspect of the invention, there is provided a pharmaceutical composition comprising a protein of the invention or a functionally equivalent variant or fragment thereof, optionally as a pharmaceutically acceptable salt, in combination with a pharmaceutically acceptable carrier.
The pharmaceutical composition may be in any appropriate form for administration including oral and parenteral compositions.
A process is also provided for making the pharmaceutical composition, in which a protein of the present invention or a functionally equivalent variant or fragment thereof is brought into association with a pharmaceutically acceptable carrier.
According to a further aspect of the invention, there is provided a protein of the invention or a functionally equivalent variant or fragment thereof for use as a pharmaceutical.
According to a further aspect of the invention, there is provided the use of a protein of the invention or a functionally equivalent variant or fragment thereof in the manufacture of a medicament for the treatment of an HCV infection.
The ability of a protein of the invention or a functionally equivalent variant or fragment thereof to bind to HCV permits the use of the protein or a functionally equivalent variant or fragment thereof as a diagnostic for HCV infection, for example in an ELISA or RIA.
A soluble form of the protein could, for example, be used in an ELISA form of assay to measure neutralising antibodies in serum.
According to a further aspect of the invention, there is provided an assay for HCV antibodies in a serum sample comprising the step of allowing competitive binding between antibodies in the sample and a known amount of an HCV protein for binding to a protein of the invention or a functionally equivalent variant or fragment thereof and measuring the amount of the known HCV protein bound.
Preferably, the protein of the invention or functionally equivalent variant or fragment thereof is immobilised in a solid support and the HCV protein, which may suitably be E2 HCV envelope protein, optionally recombinant E2 protein, is labelled, suitably enzyme labelled.
In an assay of this form, competitive binding between antibodies and the HCV protein for binding to the protein of the invention results in the bound HCV protein being a measure of antibodies in the serum sample, most particularly, neutralising antibodies in the serum sample.
A significant advantage of the assay is that measurement is made of neutralising antibodies directly (i.e those which interfere with binding of HCV envelope protein to the cellular receptor). Such an assay, particularly in the form of an ELISA test has considerable applications in the clinical environment and in routine blood screening.
Also, since the assay measures neutralising antibody titre, the assay forms a ready measure of putative vaccine efficacy, neutralising antibody titre being correlated with host protection.
In a further aspect of the invention, there is provided a diagnostic kit comprising the protein of the invention or a functionally equivalent variant or fragment thereof. Preferably the kit also contains at least one HCV labelled HCV protein, optionally enzyme labelled.
The protein of the invention or a functionally equivalent variant or fragment thereof may be used to screen for chemical compounds mimicking the HCV surface structure responsible for binding to the HCV receptor.
According to a further aspect of the invention, there is provided a method for screening chemical compounds for ability to bind to the region of HCV responsible for binding to a host cell, comprising measuring the binding of a chemical compound to be screened to a protein of the invention or a functionally equivalent variant or fragment thereof.
This aspect of the invention encompasses the products of the screening process whether alone, in the form of a pharmaceutically acceptable salt, in combination with one or more other active compounds and/or in combination with one or more pharmaceutically acceptable carriers. Processes for making a pharmaceutical composition are also provided in which a chemical compound identified by the process of the invention is brought into association with a pharmaceutically acceptable carrier.
The chemical compound may be an organic chemical and may contain amino acids or amino acid analogues. Preferably however the chemical compound is a polypeptide or a polypeptide which has been chemically modified to alter its specific properties, such as the affinity of binding to the protein of the invention or a functionally equivalent variant or fragment thereof or its stability in vivo.
At present, the only available animal model is the chimpanzee, which is a protected species. Experiments on such animals pose a number of difficulties which together result in a very considerable expense (a one year experiment with one chimpanzee can cost $70,000). Compared to this, a mouse model would be far more acceptable. Unfortunately, as described below the HCV receptor, whilst ubiquitous in humans and found in chimpanzees, is absent in other mammals. A transgenic mammal, for example a mouse, carrying the HCV receptor on the cell surface would be of great benefit to HCV research and the development of vaccines.
According to a further aspect of the invention, there is provided a transgenic non-human mammal, suitably a mouse, carrying a transgene encoding a protein of the invention or a functionally equivalent variant or fragment thereof.
The transgenic animal of the invention may carry one or more other transgenes to assist in maintaining an HCV infection.
There is also provided a process for producing a transgenic animal comprising the step of introducing a DNA encoding a protein of the invention or a functionally equivalent variant or fragment thereof into the embryo of a non-human mammal, preferably a mouse.